Method for determination of the direction to an object to be surveyed by selecting only a portion of image information depicting the object for such direction determination

ABSTRACT

An image depicting an object is recorded for the purpose of measuring the direction to said object, after which the object is to be surveyed. In order to achieve an optimum stability for rapid changes to the object&#39;s position, image sensors are used to analyze or download only a part of the available pixels. A selection of the analyzed image information, as above, uses information about the required measurement accuracy and the time performance of the image sensor. According to the invention, the limitation of the downloaded information can be achieved by the selection of a partial region of the image using the combination of a sub-sampling with a sub-windowing. A selection of image points for downloading within the partial region of the image can thus be achieved by the use of the determined omitted image information.

The invention relates to a method for determining the direction to anobject to be surveyed and a computer program product and a computer datasignal.

BACKGROUND

In many geodetic problems or applications, it is required to determine,from a detection point, the direction to an object point, such as, forexample, the azimuthal angle and angle of elevation to a furtherreference point or the compass direction. Such problems are classicaltasks of geodesy.

In order to make an object point or an object to be surveyed detectableand surveyable, this object point is distinguished from other points inspace, for example by virtue of radiation being actively emitted by it.

Another possibility for distinguishing an object point is to increasethe directed reflectivity in the object point, for example by mountingone or more reflectors, for example a corner cube with its inversioncentre on the point or in a defined environment of the point.

A further example for distinguishing an object point is its definitionas a position relative to a known object form, such as, for example, afixed target, or relative to an edge/corner/centre/centre of gravity ofan object.

From the detection point, a defined solid angle element or field of viewof a detector, which contains or should contain the object point, isdetected and recorded by a sensor so that monitoring is possible. If theobject point is present within the monitored solid angle element, thedistinguishing of the object point leads to a pattern on the sensor byvirtue of an image. This pattern specific to the object is focussed onthe detector in a direction-dependent manner with a certain bearing orposition. This position of the pattern on the sensor permits acalculation of the direction of the object point relative to thedetection point, it being possible, if required, to include additionalinformation.

An example of such an image which can be used for directiondetermination is the focused image of the object point and its definedenvironment on a position sensitive device (PSD) or image sensor withthe use of an objective or of a diffractive optical system. Anotherexample is imaging with infinite focus, which directly assigns adirection-dependent position on the sensor to received object rays. Inthis example, the divergent radiation emitted by an object point isfocussed to give a pattern having approximately circular symmetry on thesensor.

The position of the pattern is determined by the sensor or evaluationelectronics and converted into the sought direction of the object pointrelative to the detection point, it being possible, if required, to useadditional information about object properties, object distance anddetector properties.

As a suitable sensor which permits position determination, it ispossible to use, for example, a PSD as an individual sensor or an imagesensor as a matrix of individual sensors, so-called pixels or imagepoints. The latter has the advantage that any troublesome stray light isdistributed over the individual sensors or pixels of the image sensor,and the utilisation of the sensor dynamics and the signal/backgroundratio are more advantageous than with the use of only one individualsensor.

However, a disadvantage of the use of image sensors is the considerablyincreased time requirement for reading out and evaluating the pixels incomparison with the use of only one individual sensor. For example, aVGA image sensor having 640×480 pixels requires a time which is 307,200times greater in comparison with the use of an individual sensor.

In the determination of the direction to an object or an object point,problems due to an increased time requirement for reading out andprocessing the sensor signal are encountered with the use oftwo-dimensional sensors, which is advantageous because of the stabilityto interfering radiation, so that a comparatively low measuringfrequency of the direction determination results.

The direction determination can be divided into two problems dependingon the application:

Static measuring task—Here, the object point is immobile or has a changeof direction relative to the detector which is negligible with respectto required accuracy and measuring frequency of the directiondetermination.

Dynamic measuring task—Here, the change of direction from the objectpoint to the detector is not negligible. In the dynamic measuring task,problems arise if the change of the direction to the object point duringthe evaluation of the measurement is so great that the object point isoutside the field of view of the detector during the subsequentmeasurement. If a plurality of measurements follow one another, thedirection from the object point to the detector may change in the courseof the measurements, for example by a random or involuntary movement ofthe object point. Such changes, which may be repeated, give rise toproblems in the direction determination if the object point leaves thefield of view of the detector.

In this case, tracking of the field of view, possibly also performedautomatically, for example for target tracking, becomes more difficult.Under unfavourable circumstances, tracking based on the directionmeasurement and with the aim of detecting the object point again can nolonger be carried out, so that the measurement may have to be stoppedunder certain circumstances.

Optimization of the stability of the direction measurement to rapidchanges in the direction is therefore advantageous. However, a specifiedaccuracy of measurement of the direction measurement must be reached.

A special case of the direction measurement considers accuracies ofmeasurement which are greater than or equal to the field of view angleof the detector. The measuring task therefore now consists in thedecision or verification that the object point is within the field ofview of the sensor. This is sufficient, for example, for tracking theobject point.

A high measuring frequency—adapted if required—leads to a highertolerance of the regulation to rapid changes of direction and istherefore also advantageous in this special case.

High measuring frequencies are also advantageous in the case of thestatic measuring task, since, in the case of the rapid measurement, aplurality of individual measurements can be gathered within the timedetermined by the application and an increase in the accuracy of themeasurement is thus possible. Moreover, brief strong disturbances, whichcan be eliminated in the case of rapid measurement, occur in the eventof a disturbance of the measurement by turbulent air flows (heatstriae).

SUMMARY

An object of the present invention is to provide a method whichstabilizes direction measurements to changes of direction, whilemaintaining the required accuracy of measurement.

A further object of the present invention is to permit tracking based ona direction measurement, even in the case of relatively high angularvelocities or angular accelerations of objects to be detected.

The invention relates to a method for determining the direction to anobject point, an image sensor or an array of individual sensors beingused for reasons of stability to stray light.

In the case of special types of image sensors, such as, for example,CMOS image sensors, it is possible to access individual image points orpixels directly. Such image sensors firstly permit the limitation ofthe—for example square—evaluated image field of the sensor in the formof so-called “subwindowing”. Associated with the reduction in the numberof pixels read out is a shorter time during reading out and subsequentlyprocessing the pixel data.

Secondly, in the case of such sensors, a time gain can also be achievedby so-called “subsampling”. This is the reading out of, for example,only every 2^(nd) (3^(rd), 4^(th), . . . ) column and/or only every2^(nd) (3^(rd), 4^(th), . . . ) row of the image sensor array.

According to the invention, optimization of the stability of thedirection determination to changes in the direction is effected by thechoice of that combination of subsampling and subwindowing which isoptimum in this context on the basis of the required accuracy ofmeasurement and on the basis of the sensor timing. For this purpose,information about both the required accuracy of measurement and the timebehaviour of the image sensor is used. The optimization can of coursealso be effected with specification of one or more secondary conditions,for example limits for the measuring frequency.

Subsampling and subwindowing are combined so that a quantity of pixelsis selected within a partial region of the image detected by thedetector, so that no pixels are taken into account outside the partialregion. The parameters for selecting the partial region and theparameters for selecting the pixels within the partial region areoptimized while maintaining the necessary accuracy of measurement.

The method according to the invention has advantages over puresubwindowing or pure subsampling since the optimization of thesubwindowing as a function of time i.e. for achieving a high measuringfrequency, would mean a maximum reduction of the area of detection. Onthe other hand owing to the evaluation of the total detection area, puresubsampling is, with regard to the minimum number of pixels to beevaluated, substantially greater than the method according to theinvention, resulting either in lower measuring frequencies with the sameaccuracy of measurement or lower accuracies of measurement with the samemeasuring frequency.

Below, the reading out of only every N th column (or N th row) isdesignated as N fold column subsampling (N fold row subsampling).

In both cases, only a portion of the image information recorded by meansof the image sensor is used. In the simplest case, this consists in theselection of a portion of the pixels whose content will be read out.However, it is also possible to form aggregates of a plurality ofpixels, for example in the form of the combination to givesuperstructures of pixels.

In a step upstream of the actual direction measurement, the conditionsor parameters of the image recording and image evaluation can beestablished. On the basis of object size, object distance and/or desiredaccuracy of measurement, it is decided whether/and which, columnsubsampling and whether/and which row subsampling can be carried out.Here, the pattern position which permits the calculation of thedirection to the object point should also be capable of being determinedsufficiently accurately by means of subsampling. This applies inparticular if the pattern is generated by a focused image of a complexobject point environment. The position of the image of a measuring markon a sensor can be extracted sufficiently accurately only if this imageincludes a relatively large number of pixels—dependent on the complexityof the marking. An example of an estimation of the accuracy of themeasurement for a simple pattern is outlined below, the descriptionbeing given only for the row direction of the sensor. The procedure inthe case of column direction is effected analogously.

The pattern contains positions recognisable in the horizontal (row)direction of the sensor. N_(T). These are typically light-dark ordark-light transitions. Furthermore the recognisable positions generallylie at the edge of the pattern, i.e. the recognisable positions arefrequently not part of the texture of the pattern.

From object size and object distance, it is possible to calculate thesize or the pattern on the sensor. If the recognisable positions of thepattern are not oriented on the pixel grid, which is scarcely alimitation for practical applications, the number of pixels on the edgethereof can therefore be estimated and N_(T) thus determined. For theerror of the position determination E_(p) of the pattern, the followingproportionality relationship is obtained.

$\begin{matrix}{E_{P} \propto \frac{T}{\sqrt{N_{T}}}} & (1)\end{matrix}$where G specifies the insensitive gap between two pixels. For thispurpose, it is also necessary to take into account the error whichresults from the signal noise.

Without subsampling, G is the distance between the sensitive areas ofadjacent pixels, from which a filling factor <1 results for G>0. Withsubsampling, the area of the pixels which have not been read out and arepresent between the pixels read out is added to this pixel spacing, thesubsampling also reducing N_(T).

The proportionality factor in equation (1) can be theoretically derivedor determined on the basis of measurements for simple patterns.

N-fold subsampling can be determined with the maximum N which stillensures the desired accuracy of the measurement of the directionmeasurement.

With the ideal choice of subwindowing, the previously made choice ofsubsampling must be taken into account. In addition, it may beadvantageous to include the size of the pattern in the optimization, italso being possible, for example, to estimate said size from the objectdistance.

The size of the field of view is adjusted so that a maximum angularacceleration of the object point which occurs between two directionmeasurements can be tolerated, i.e. the size of the field of view ischosen so that, in spite of the angular acceleration, the object pointis still present in the field of view of the detector during the secondmeasurement.

The term of “geodetic surveying” or “geodetic application” is alwaysintended to designate generally measurements which include adetermination or checking of data with spatial reference. In particular,it is also to be understood as meaning all applications which areeffected in association with the use of a geodetic instrument orgeodetic measuring device. This applies in particular to theodolites andtotal stations as tacheometers with electronic angle measurement andelectrooptical telemeter. Similarly, the invention is suitable for usein specialised apparatuses having a similar functionality, for examplein military aiming circles or in the monitoring of industrial structuresor processes or machine positioning or guidance.

BRIEF DESCRIPTION OF THE DRAWINGS

The method according to the invention is described in more detail belowpurely by way of example with reference to working examples shownschematically in the drawing.

Specifically,

FIG. 1 shows the diagram of a possible use of the method for surveying;

FIG. 2 shows the diagram of the recording of an image with a pattern bymeans of an image sensor;

FIG. 3 shows the diagram of a selection of image information bysubwindowing;

FIG. 4 shows the diagram of a selection of image information bysubsampling;

FIG. 5 shows the diagram of a selection according to the invention ofimage information by a combination of subwindowing and subsampling;

FIG. 6 shows the diagram of the conditions in the case of a dynamicmeasuring task and

FIG. 7 shows the diagram of a transformation model for derivingdirection information from the position of a pattern.

DETAILED DESCRIPTION

FIG. 1 shows a possible use of the method according to the invention forsurveying. By means of a total station as a geodetic measuring device 1,reference points which are recognisably characterized by a plumbingstaff having a reflector as object 2 are surveyed on a building site.The image sensor 1 a integrated in the measuring device 1 has a sensorfield of view 3 in which the object 2 to be surveyed should be present.The direction to this object 2 is determined. Although in this figurethe sensor field of view 3 is shown as being rectangular purely by wayof example, it can also have other shapes.

FIG. 2 shows the diagram of the recording of an image 4 with a pattern 6by means of an image sensor. The image 4 recorded by the image sensorregisters the object 2 to be surveyed. This image 4 is recorded by thesensor by an array 5 of pixels and converted into signals which can beelectronically evaluated. A pattern 6 on the array 5 corresponds to theobject 2 to be surveyed. This pattern 6 and the pixels coordinated withit can be identified, for example, on the basis of the transition fromlight to dark. However, the reading out of all pixels 5 a of the array 5requires a certain time, which determines the achievable frequency ofthe image processing. For determining the direction of the object 2,however, it is sufficient to know the bearing of the sample 6 in theimage 4 or on the array 5 so that not all pixels 5 a of the array 5 arerequired to the full extent. While a complete read-out is alwayseffected in the case of CCD cameras the individual pixel 5 a can beselectively read out in the case of other designs, such as, for example,CMOS cameras, so that a use tailored to the image content required forthe direction determination can be realised.

FIG. 3 shows the diagram of a selection of image information bysubwindowing. The pattern 6 of the object detected in the image 4 isrecorded by a cohesive portion of the pixels of the image sensor, thisportion defining a window as partial region 7 a of the image 4. Thismeans that only a part of the image defined by the field of view of thesensor is evaluated, the evaluation, however, using all available pixelsin the partial region 7 a considered. The reduction of the pixels usedcan be effected even during a recording by using only a part of thepixels at all for recording—for example on the basis of hardwaremeasures—or in the determination of the position of the pattern byreading out only a part of the image information available in principle.

FIG. 4 shows the diagram of a selection of image information bysubsampling. Here, pixels 5 a are excluded from use according to acertain scheme so that only the content of a portion of pixels 5 a isused. In this example, only every 2nd pixel 5 a is used in each row andin addition the content of every 2^(nd) row is completely neglected.Moreover, the pixels 5 a used are offset relative to one another row byrow. The pattern 6 of the object detected in the image 4 is recorded bya portion of the pixels 5 a of the image sensor, this portion coveringthe entire image 4 defined by the field of view of the sensor. Thepixels 5 a available in principle are not completely used. In comparisonwith the use of all pixels 5 a this is a recording with a coarser gridwhich corresponds to an image sensor having a reduced filling factor.The selection of the pixels 5 a which is shown is only one example.According to the invention, a wide range of further schemes may be used.In particular, selection methods without row-by-row offset (columnand/or row subsampling) or selection methods with non-periodic sequencesor aggregates of pixels 5 a can also be used.

FIG. 5 shows a selection, according to the invention, of imageinformation by a combination of subwindowing and subsampling. In thecase of this selection, the approaches shown in FIG. 3 and FIG. 4 arecombined so that only a partial region 7 b of the image 4 is used forthe determination of the position of the pattern 6. In this partialregion 7 b, not all pixels available in principle for an evaluation areactually used, but a selection of the pixels is made according to ascheme. This selection of image information thus follows a two-stageapproach. Firstly, only a partial region 7 b of the image is used atall. Secondly, not all available pixels are evaluated within thispartial region 7 b. According to the invention, other combinations ofsubwindowing and subsampling can also be used over and above thisexample. In particular, it is also possible to use a plurality ofpartial regions with different internal selection, it also beingpossible for these partial regions to overlap.

FIG. 6 illustrates, by way of example, the calculation of the optimumimage resolution of a sensor having square pixels—as shown in FIG. 2 toFIG. 5—and the same velocity requirement in both sensor directions. Theprocedure can easily be generalised to include rectangular pixels and/ordifferent velocity requirements.

Let the image resolution be N_(P)×N_(P) pixels. The time requirementT_(M) of the direction measurement is found from the image resolution tobe typically the 2^(nd) degree polynomial having the coefficient C_(n).T _(M) =C ₂ N ² _(P) +C ₁ N _(P) +C ₀  (2)

The pattern 6 is present on a sensor region with N_(P)×N_(P) pixels. Inthis example, the limits thereof are assumed to be a circle having aradius R_(M). If it is wished to ensure a continuous directionmeasurement during the measuring task, the pattern 6 is not permitted toleave the sensitive region during the measuring time T_(M). Thus, themaximum velocity of the pattern 6 on the sensor is:

$\begin{matrix}{V_{Max} = {\frac{D}{T_{M}} = \frac{\frac{N_{P}}{2} - R_{M}}{{C_{2}N_{P}^{2}} + {C_{1}N_{P}} + C_{0}}}} & (3)\end{matrix}$

The optimum subwindowing maximises this velocity:

$\begin{matrix}{N_{P,{Opt}} = \frac{{2\; R_{M}C_{2}} + \sqrt{{4\; R_{M}^{2}C_{2}^{2}} + {C_{2}C_{0}} + {2\; R_{M}C_{2}C_{1}}}}{C_{2}}} & (4)\end{matrix}$

If the image resolution N_(P,Opt)×N_(P,Opt) is chosen, this gives thegreatest possible velocity of the pattern on the sensor which stillpermits successive measurements. If the pattern 6 has moved the distanceD on the sensor during the measuring time, the measurement can still becarried out at the initially central bearing of the pattern 6 before thefield of view of the detector has to be adjusted for the nextmeasurement. If the value of N_(P,Opt) exceeds the number of pixels in asensor direction, e.g. N_(P,Opt)>number of pixels in the row, takinginto account possible subsampling, the sensor must be adjusted in thisdirection without subwindowing. In this example, this means that, ofrows which provide the possible row subsampling, all pixels whichprovide the possible column subsampling are evaluated. This would alsobe the procedure for the case of C₂=0.

If only a continuous adjustment of the field of use is to be effected,it is often also possible to determine the position of the pattern 6comparatively coarsely, for example with a permissible error ofmeasurement corresponding to half the field of view of the detector, ifonly the centre of the pattern is in the field of view of the sensor.This means that only a part of the area of the pattern 6 is in theevaluated sensor region. In this problem, the maximum permissiblevelocity of the pattern 6 on the sensor is

$\begin{matrix}{V_{Max} = \frac{\frac{N_{P}}{2}}{T_{M}}} & (5)\end{matrix}$and hence the optimum resolution N_(P,Opt)×N_(P,Opt) of the evaluatedimage region is:

$\begin{matrix}{N_{P,{Opt}} = \sqrt{\frac{C_{0}}{C_{2}}}} & (6)\end{matrix}$

Once again, if N_(P,Opt) is greater than the number of pixels which canbe evaluated—taking into account the subsampling—in a sensor direction,all these pixels are evaluated. The same applies to both sensordirections if C₂=0.

In the following figures, a possibility for calculating the desireddirection information from the position of the pattern on the imagesensor is outlined by way of example.

FIG. 7 shows the transformation model for the transformation of an imagecoordinate of a point q of the pattern as a polar angle of a detectedobject having an object point Q. By means of this transformation model,it is possible in principle to derive the position or the direction ofan object point from the position of the pattern.

In order that the polar angle of an arbitrary object point Q within thefield of view of the sensor can be determined on the basis of itsposition in the pattern or in the image 4 which is detected by the imagesensor, and hence on the basis of its image coordinate, a mathematicaldescription of the imaging of the object present in the field of view ofthe sensor as a pattern—or of an object point Q as a corresponding pointq in the pattern—in the image form must be known. Below, thetransformation of points in the image coordinate system x, y, z into theobject coordinate system X, Y, Z is to be described with reference toFIG. 7. The Z axis points in the direction of the zenith and represents,for example, the vertical axis of a geodetic measuring instrument, andthe X axis is formed, for example, by the tilting axis.

For a simplified transformation with limited accuracy, it is possible tomake the following assumptions, a geodetic instrument which correspondswith regard to its systems of axes and its basic design to a theodolitebeing used by way of example as a starting point:

-   -   The projection centre 81 of the focusing of the objects detected        within the field of view of the sensor onto the image sensor is        at the point of intersection of vertical axis and tilting axis.    -   The tilting axis is perpendicular to the vertical axis.    -   The optical axis 82 and the theodolite axis 83 intersect at the        projection centre 81.

Here, the optical axis 82 is defined as the axis through an optical unitand hence substantially that axis which passes through the centres ofthe lenses. The theodolite axis 83 is defined as that axis relative towhich the angles of rotation about the vertical axis and the tiltingaxis are measured. This means that the point of intersection of thetheodolite axis 83 with the image sensor in the case of a two-bearingmeasurement points exactly to that object point Q of the object which isto be surveyed. This corresponds to the sighting axis with respect tothe crosshairs in the case of optical theodolites.

However, it is also possible not to start from these assumptions but toextend the transformation appropriately, for example axis errors—inparticular an axis offset or an axis skew—being included in thetransformation. This ensures a further increase in the accuracy of thetransformation and is therefore particularly suitable in the case ofgeodetic measuring instruments of the highest precision class.

The calculations are limited to the focusing of an object point Q in asuperior coordinate system, which is horizontal and the origin of whichis at the projection centre 81, into the image plane of the image 4. Thetransformation into an arbitrary coordinate system can be carried out bymeans of displacement and rotation via the known Helmert transformationwith a scale equal to one.

The transformation model for the transformation of a recorded imagecoordinate into an object coordinate is as follows:

$r_{q} = {r_{P} + {T_{0} \cdot \left( {\frac{1}{m} \cdot T_{{H\; z},v} \cdot R_{Inc} \cdot r_{Q}} \right)}}$where

-   -   r_(Q) is the object vector 84 of the point Q in the system        (X,Y,Z).    -   r_(q) is the vector of a point q of the pattern, i.e. of the        copy of the object point Q on the image 4, measured in the image        coordinate system x,y,z. The x and y components are determined        by the recorded image coordinate 9. The z component corresponds        to the chamber constant c which is defined as the distance of        the image sensor and hence of the image 4 from the projection        centre 81. The chamber constant changes with the position of a        focusing lens of the optical unit and hence with the focused        object distance.    -   r_(p) is the image origin vector which describes the point of        intersection p of the optical axis 82 with the image plane 4.    -   m is the imaging scale.    -   R_(Inc) is the rotation matrix which relates the tilted        theodolite plane and the horizontal plane.    -   T_(Hz,v) is the transformation matrix which describes the        orientation of the theodolite axis 83 based on the horizontal        angle H, the vertical angle V and the corrections of the axis        errors.    -   T₀ is the matrix for modelling the optical distortions.

FIG. 7 shows the above transformation of the object point r_(Q) from thesuperior coordinate system X, Y, Z into the image coordinate system x,y, z. By means of the measured angle of inclination, the horizontalangle H, the vertical angle V and the axis corrections, it is possibleto map the object point vector r_(Q) into the system of the imagesensor. The deviation of the optical axis 82 from the theodolite axis 83and the optical distortions are corrected by means of suitabletransformations and calibrations.

Approaches from photogrammetry, such as, for example, the modellingknown from the prior art and attributable to Brown or Bayer, aresuitable here. In the case of narrow-angle systems, the correction canbe modelled by a simple affine transformation.

A further example of a conversion of the position of the pattern on theimage sensor into direction information is the infinite focusarrangement. Here, the image sensor is mounted in the focal plane of anobjective. If a beam of sufficiently small divergence emanates from theobject point, the position of the—often circular—pattern resultingtherefrom corresponds directly to the direction relative to the firstprincipal point of the objective.

In the figures, the steps of the method, buildings and instruments usedare shown purely schematically. In particular, no size relationships ordetails of the image recording or image processing can be derived fromthe diagrams. The points shown only by way of example as pixels alsorepresent more complex structures or a larger number of pixels in animage sensor.

1. A method for determining the direction to an object to be surveyed,using a geodetic measuring instrument comprising an image sensor, thefield of view of which sensor detects at least part of the object to besurveyed, the method comprising the following acts: recording an imagewith image information using the image sensor, the image depicting anobject and the object representing a pattern in the image which can becoordinated with the object, wherein the position of the pattern withinthe image permits a determination of the direction to the object;determining the position of the pattern within the image; and derivingdirection information coordinated with the object from the position ofthe pattern, the direction from a detection point coordinated with theimage sensor to the object being determined, wherein only a portion ofthe image information is selected and used for the directiondetermination, and wherein the portion of the image information isselected with regard to a specified accuracy of measurement.
 2. A methodaccording to claim 1, further comprising periodically omitting pixels,the periodicity being chosen so that the local resolvability of theposition of the pattern permits the determination of the direction tothe object with the specified accuracy of measurement.
 3. A methodaccording to claim 1, wherein the act of recording the image furthercomprises subwindowing as a selection of a partial region of the imagesensor and subsampling as a specific omission of pixels within thepartial region.
 4. A method according to claim 1, wherein the act ofderiving the direction information further includes verifying that theobject is positioned at least partly within the field of view of thesensor.
 5. A method according to claim 3, wherein the act of subsamplingis performed before the act of subwindowing.
 6. A method according toclaim 3, wherein the selection of the partial region is made during thesubwindowing on the basis of at least one of the following variables:object size; object distance; desired accuracy of measurement;dimensions of the pattern; and expected or measured maximum angularacceleration.
 7. A method according to claim 3, further comprisingomitting a selection of pixels during the subsampling on the basis of atleast one of the following variables: object size; object distance;desired accuracy of measurement; dimensions of the pattern; and expectedor measured maximum angular acceleration.
 8. A method according to claim1, further comprising omitting columns and/or rows of the image sensorduring the subsampling.
 9. A method according to claim 8, wherein theomitted columns and/or rows are in the form of a rectangular partialregion of the image sensor.
 10. A method according to claim 3, whereinthe act of subsampling includes omitting pixels in a regular orstochastic sequence.
 11. A computer program product having a programcode, which is stored on a machine-readable medium, configured to carryout the method according to claim 1.